If everyone is rational, why does everyone else make such stupid decisions?
It must be that, because they are closer to their own decisions, they have more information than I have when I'm over here judging them. And not just any information, but information that makes the difference between me seeing their choice as wise or foolish. This is probably the most-pertinent information, and it must be the least-obvious information, as well.
This is bad news for my own decision making. It means that I can't just cut off my information-gathering at an arbitrary point, because the pertinent information may still be outside my grasp. As for rules-of-thumb, their usefulness might be enhanced by this view (why do a lot of intensive information gathering when I can just use the heuristic?) or diminished (decisions are too important to rely on a rhyming couplet devised by someone who maybe didn't gather all the information first).